For VSYNC and LS_BF_VS interrupts, please refer to the Section 9.5.3 of the P1022 Reference Manual:

If enabled, the VSYNC status bit will always be asserted with the first vsync pulse cycle. With this interrupt, the host can always observe the beginning of a new frame. Beside this, another interrupt, the LS_BF_VS (lines before vsync) can be used to set a deadline for the host to program the pointers for the current next frame. This interrupt is asserted at user-specified lines (set by the LS_BF_VS threshold, Section 9.3.17, “THRESHOLDS Register”) before the vertical front porch (FP_V). The host can program the pointer registers after it detects the LS_BF_VS interrupt.

Figure Figure 9-36 shows a diagram on how these two interrupts can be used to synchronize the host and the DIU.

UNDRUN interrupt.Under run exception interrupt is asserted when display needs data and output buffer filling is lower than or equal to the OUT_BUF_LOW threshold. See please the Section 9.5.5